Micromachined inferential opto-thermal gas sensor

ABSTRACT

A micromachined integrated opto-thermal sensor having a rapidly intensity varying or pulsing light source, an interference filter, shadow masking or reflective blocking of light from thermal sensors, or differential operation, a gas cavity into which the detected gas can flow into via a channel or filter, and thermal detector elements to sense the heating of the gas caused by the absorption of light at a particular wavelength by the specific gas to be detected. Another version of the sensor is one with a dual cavity. One cavity contains the gas to be detected and the other cavity is sealed from the ambient environment and contains no gas. Signals from the detectors from the cavities are subtracted from each other resulting in the elimination of a fixed signal due to radiation impinging the detectors.

This application is a divisional of copending application Ser. No.08/846,724, filed on Apr. 30, 1997.

BACKGROUND

The invention pertains to gas sensors and particularly to toxic gassensors. More particularly, the invention pertains to micromachinedintegrated circuit gas and fluid sensors.

Related art devices for sensing toxic gases such as CO, CO₂, NO, NO₂ andVOCs generated by combustion processes have been based on sensorsindicating changes in metal oxide film conductivity, chemiluminescence,fluorescence, various forms of IR absorption, and so forth. Thesesensors have been either too costly, unstable, or insensitive to meetthe requirements of a low-cost, reliable toxic gas sensor. Their sensingsuch toxic gases in concentrations that are commensurate with the levelsat which they can be harmful to health and life is difficult, especiallyif it is to be done via low-cost, affordable and reliable sensors. Oftenthe older gas engines or heaters, operated by budget-minded users, aremost likely to be a source of toxic gases which endanger these users andothers. These users are the ones most unlikely to buy toxic gasindicators, unless someone manages to bring affordable and appropriatetechnology to them.

Optoacoustic gas sensors sense low concentrations of gases by inducing agas temperature variation by narrow-band modulated illumination at awavelength which the gas absorbs. The modulated temperature signal isnot sensed directly, but a closed or nearly-closed gas sample cell isused which converts the small gas temperature signals into a pressuresignal, which is detected by a microphone. A closed or nearly-closed gascell makes it difficult for gas to enter and exit the gas cell.

SUMMARY OF THE INVENTION

Direct sensing of the gas temperature modulation signal, which is calledoptothermal sensing, removes the need for a closed or nearly-closed gascell. The direct sensing of the gas temperature signal is handicapped bya lack of a suitably sensitive (i.e., nanodegree sensitivity) andfast-response gas temperature sensor. The use of a micromachinedthermoelectric sensor array does allow suitably sensitive, fast responsedetection of the small gas temperature modulation signal. Such arraysare conveniently fabricated by silicon micromachining.

The present invention provides a new, useful, low-cost and reliabledirect sensing of the gas temperature signal of the present gas, andalso provides the inferred indication of the presence of a toxic gas orobjectionable constituents of combustion products. It is not necessaryto directly measure the toxic or objectionable gases, if one canidentify a phenomenon that would indicate or infer their presence with ameaningful probability level. The present sensor thus provides morecompact, reliable, affordable detection than direct NDIR sensing oftoxic gases. It also provides additional detection/alarm protectionagainst high CO₂ or other gas concentrations by direct sensing.

The sensor takes advantage of the indirect indication of toxiccombustion products, such as CO, NO_(x) and VOCs via CO₂ detection, anda low-cost, integrated gas sensor design is thus made available at areasonable price to meet the toxic gas sensing needs of users ofunvented space heaters (or kitchen stoves) and the needs of automobiledrivers that wish to detect exhaust fumes from cars or near them.

Carbon dioxide (CO₂) indicates the presence of objectionableconcentrations of combustion products. CO₂ is generated by combustionprocesses, in concentrations that are 10 to 100 times higher than thoseof CO, NO_(x) or VOCs. Yet one can measure CO₂ at concentration levelsthat are 3 to 30 times lower than the above-noted gases, especially viaNDIR. Combustion products, especially those from gasoline or diesel fuelare known to consist of 5-15% CO₂ , 10-20% H₂ O, 0-10% O₂, 70-80% N₂,0.001 to 0.4% NO_(x), 0.001 to 0.2% CO (CO in worn or maladjustedautomotive engines may be up to 2%), and 0.001 to 0.3% hydrocarbons(HC), i.e., CO₂ concentrations always predominate. Still, dilution ofexhaust gas of the car in front is expected to be 10 to 1000-fold beforereaching the cabin air intake of the following car, so that the CO₂concentration is likely to be only 0.005 to 1.5%, which is measurable,while the toxic gas concentration is in the 0.0001 to 0.04% range. Thelatter concentrations are much more difficult to measure, and especiallyso with low-cost sensors, which would often not begin to sense thosegases in spite of being present in concentrations that cause discomfortor adverse health effects.

The integrated design of the present sensor enhances itsmanufacturability and affordability. The gas cell, thermal detector andoptical filter are integrated into one compact micromachined unit whichis of lower cost, i.e., more affordable and more widely applicable thanhigher cost sensors. Infrared radiation may be obtained from small lightbulbs, or from electrically heated microbridges (microemitters).Electronic circuits may also be integrated into the silicon material.The sensors are more compact and therefore more rugged, and overall moreuseful. The integrated opto-thermal sensor used as the detector of thepresent gas results in a more sensitive, faster response and more stabledetection. The faster response is because a closed or nearly closed gascell is not required.

The integrated sensor is 10 to 100 times smaller than the related artsensors, which makes the present system more affordable, portable anduseful. The present detectors are also 10 to 100 times less costly thanthe related-art detectors because they can be mass-produced usingsilicon micromachining.

The present highly accurate gas detector is formed from micromachinedsilicon technology thereby being much smaller than related-artdetectors.

In summary, the invention is a low-cost opto-thermal sensing system,which is a micromachined integrated sensor, which has a pulsing, heatedradiation source, an appropriate multi-layer interference filter (IF),anti-reflective (AR) film, shadow masking or reflective blocking toprevent light from impinging thermal sensors, and specially etchedsilicon wafer or masking designed to maximize the infrared red (IR) orlight of other wavelengths, to provide energy efficiency at, forinstance the 4.3 micron wavelength band of CO₂, a sample gas cavity intowhich gas can flow in and out via channels, or diffuse in and out theetch holes used earlier in fabrication to dissolve the sacrificial layerutilized to form the cavity or via a porous compressed stainless steelfrit, and a micromachined gas temperature sensor, operated insingle-output or differential-output manner.

The effect of slow ambient temperature variations on the sensor arenaturally rejected by a thermoelectric junction-pair arrangement. Theeffect of gas temperature variations caused by air and/or gas drafts maybe minimized by suitable porous baffles, and by lock-in detection. Tominimize background signals, the thermoelectric temperature sensors maybe not directly illuminated by the optical radiation, and may be coatedwith a reflective material, and may be operated in a differential mannerby placing suitable IFs between the optical illuminator and the gastemperature sensors, and a suitable gas inlet arrangement.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a micromachined inferential opto-thermal gas sensor.

FIG. 2 shows another design of an inferential opto-thermal gas sensor.

FIG. 3 is of still another design of an opto-thermal gas sensor.

FIGS. 4a, 4b and 4c are waveform diagrams of light and heat signals of athermal sensor.

FIG. 5 is a diagram of the structure of a thermal sensor element.

FIGS. 6a, 6b, 6c and 6d show a sensor operation in a differentialmanner.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a diagram that shows a basic structure of inferentialopto-thermal gas sensor 10. Silicon wafer 11 has an etched space 12 onone side. On that side having depression 12 is formed a silicon wafer 13having a set of microemitters 14 on the side adjacent to wafer 11.Formed on the surface or side of wafer 11 adjacent to microemitters 14is an antireflective (AR) coating 15. On the other side of wafer 11 is anarrow band pass interference filter (IF) 16 designed to pass onlyinfrared light having a wavelength that is the same as the absorptionwavelength (4.3 microns) of CO₂. The AR and IF coatings or films may beinterchanged in location with each other. Silicon wafer 17 is formed onfilter 16. Silicon wafer 18 is formed on wafer 17. Silicon wafers 17 and18 are etched to form a cavity 20 and channels 114. Channels 114 form apathway between cavity 20 and the ambient volume or space external tosensor 10. Gas or air 21 can diffuse or flow in and out of cavity 20 viaorifice, path or channels 114. Wafer 18 has thermal sensors 19 formedover pits 116. Microemitters 14 and thermal sensors 19 are connected tocontact pads 24. Formed on wafer 13 or 18 may be an integrated circuit(IC) or an application specific integrated circuit (ASIC) for providingelectronics 25 for controlling microemitters 14 or processing signalsfrom thermal sensors 19. Wafer 11 may be substituted with a glass plate.Even wafer 17 may be substituted with glass. In the embodimentsdisclosed below, the IF filter and the AR coating may be situated orformed on glass, also.

Radiation source 14 is a 32×32 array of microemitters that function asan infrared radiation source. Array 14 provides total emission at 4.3microns about 2.8 times that of a mini-tungsten light bulb. Cavity 20 isabout 100 microns deep×500 microns wide. Cavity 20 cannot be too smallor gas cooling at cavity surfaces would reduce the sensitivity of gassensor 10.

The thermal sensor is a 64×64 array of series-connected NiFe:Crthermoelectric sensors 19, each having two thermoelectric metallicjunctions per each 50 micron×50 micron silicon nitride microbridge, onejunction on the microbridge and one on the adjacent silicon, with 10ohms resistance per junction-pair, and a junction pair Seebeckcoefficient of 60 microvolts/degree C. The thermoelectric sensors 19 arecoated with a reflective metal layer to minimize direct absorption ofinfrared radiation. The thermal sensor has a typical microbridgeresponse time of 0.5 millisecond and a 10 Hz illumination modulation.The lock-in electronics detection system (for example, amplifier 102,power source 104 and lock-in amplifier 103 in FIG. 3 with sourceelements 94 instead of lamp 93) has a 30 second response time (i.e.,bandwidth dF=0.02 Hz). The rms. voltage noise=square root of (4KT(64×64) RdF)=2.5 nanovolt rms. and sensitivity=(2.5e-9) /(64×64×60e-6)=10 nanodegree C. rms. This allows detection of typical gastemperature signals from a CO₂ concentration of about 100 ppm.

In FIG. 1, source 14 emits light 118 and 119. Light 119 is blocked byshadow masks 113. Light 118 goes through layer 15 and wafer 11. Onlylight 118 having a wavelength that is passed by narrow band passinterference filter 16 enters cavity 20 and is able to impinge airand/or gas 21 molecules. If such gas 21 has an absorption wavelengththat is the same wavelength of light 118 that passes through filter 16and impinges gas 21, then that light 118 is absorbed by gas 21 and gas21 heats up. The increase of the temperature of gas 21 is sensed bythermal sensors 19, which output signals indicative of the presence ofgas 21. Light 118 that is not absorbed by gas 21 impinges non-thermalareas 117 and does not affect sensors 19. Little light 118 or 119 willhit sensors 19 because of shadow masks 113. Light 119 from source 14passes through film 15, wafer 11 and narrow band pass filter 16, andimpinges masks 113. Masks 113 largely block light 119 that wouldotherwise enter cavity 20 and impinge thermal sensors 19. Impingement ofsensors 19 by light 119 would cause sensors 19 to warm up and providefixed signals not indicating presence of a gas. If light 119 impingedsensors 19, electronics may be used to remove fixed signals caused bysuch light 119 and pass only true signals indicating the presence of gas21. This method of operation requires very stable electronics to removethe fixed signals. An alternative approach in FIG. 6a is to employ twoarrays, 121 and 122, of thermal sensors 19, both illuminated by the sameradiation source 120 through infrared filter 125 with one array 121exposed to gas 21 and the other array 122 not exposed to gas 21. In FIG.6b, two signals 123 and 124 from two arrays 121 and 122, respectively,may then be electronically subtracted to give a signal 127 by adifferential amplifier 126 to substantially remove the fixed signalscaused by impingement of thermal sensors or temperature detectors 19 byradiation.

FIG. 6c shows another differential approach, in which two arrays 128 and129 of thermoelectric sensors 130 in a common gas cavity 139 areilluminated by two different wavelengths obtained by a lamp 134 and twodifferent interference filters 132 and 133, such that the twowavelengths are substantially equal in intensity, but one wavelength isabsorbed by the gas 131 to be directly sensed, and the other wavelengthis not. A first electrical signal is taken from array 128, comprising afixed signal caused by impingement and absorption of radiation onsensors 130 together with a signal component dependent upon theconcentration of the gas to be directly sensed. A second electricalsignal is taken from array 129, comprising only a fixed signal caused byimpingement and absorption of radiation on sensors 130. The two signalsare taken via leads 136 and 135 respectively, to a differentialamplifier 137 shown in FIG. 6(d), producing a subtracted signal 138 inwhich the fixed signal caused by impingement and absorption of radiationon sensors 130 is substantially removed.

In the differential approaches shown in FIG. 6(a) through (d), themagnitude of the second signal may be used as a measure of the intensityof the radiation source, so that changes in the intensity of theradiation source may be detected and the signals corrected accordingly.

As in the non-differential approaches, the thermal sensors used in thedifferential approach may also be provided with radiation masks, orcoated with reflective metal layers, to minimize direct impingement andabsorption of infrared radiation.

FIG. 2 is a cross-section diagram of an opto-thermal gas sensor 70 withradiation passing in an opposite direction. Silicon wafer 46 is about5×5 millimeters (mm) square and about 20 mils thick. Wafer 46 has formedon it a heated radiation source 47 of IR radiation. Source 47 isfabricated from a high refractory material such as silicon nitride withresistive heating materials. Grooves or pits 48 are etched in wafer 46to minimize heat loss from source 47. Leads 60, about one mil thick, areattached to contacts 49 for providing an AC signal at a frequency from10 to 100 hertz to activate source 47 so as to emanate radiation 51.Attachment materials 50 are formed on the periphery of chip or wafer 46.A silicon wafer 52 about 20 mils thick is attached in a vacuum, so thatspace 55 is evacuated of air. An AR film coating 53 is formed on a firstside of wafer 52 and a narrow band pass IF multiple stack layer 54 forpassing 4.3 microns of light is formed on a second side of wafer 52. ARfilm layer 53 is about 2 to 6 films of quarter wavelength thicknesses ofalternating materials having different indices of refraction. IF layer54 is a stack of half wavelength films of alternating materials havingdifferent indices of refraction. Wafer 52 is brought into proximity withwafer 46 upon contact of attachment materials 50 on wafer 46 at aperipheral surface of wafer 52 to form an evacuated thermally isolatingspace 55.

The heated radiation source 47, being within 1 to 2 microns of the solidSi substrate, has a fast response, is modulated at as high a frequencyas possible (typically 10 to 100 hertz) and fills the cavity with light,which is essential to obtain high sensitivity. If light source 47 wereto be an incandescent mini-tungsten filament lamp, the maximum pulserate of the AC excitation signal would be about 10 hertz. Increasedfrequency results in better sensitivity since low-frequency electronicnoise is less present. The present integrated circuit light source 47can effectively be cycled or pulsed up to 100 hertz which results inimproved sensitivity of sensor 70.

A silicon detector wafer 69 is formed with a first surface on 4.3 micronnarrow band pass optical interference filter 54 and silicon wafer 52.Wafer 69 has grooves or pits 72 formed or etched on a second surface ofwafer 69 for reflection of radiation 51 and for improved thermal contactof elements 71 with the gas. A thermo-electric (TE) temperature sensoror detector layer 73 is formed on wafer 69. Temperature sensitiveelements 71 are formed over pits 72. Elements 71 are coated withreflective metal to minimize direct absorption of infrared radiation.Temperature insensitive and radiation 51 transparent portions 74 ofsensor layer 73 are formed on the non-etched portions of the secondsurface of wafer 69. Electrical contacts 75 are formed on detector layer73 for electrical signal transmission to and from layer 73 via leads 60.Attachment materials 115 are formed on the periphery of layer 73 and thesecond surface of wafer 69. A top cap silicon wafer 77 is formed andattached to form cavity 78. The attachment is such that at one or morevias, channels or holes 79 are formed such that gas and/or air can entercavity 78.

The functioning of opto-thermal gas sensor 70 includes the emission offluctuating or pulsing radiation 51 having an IR component. Light 51goes through AR layer 53 and through wafer 52 to IF layer 54. A portionof light 51 is filtered out by narrow band pass film layer 54 whichpasses only light having a wavelength of, for example, 4.3 microns (forCO₂ detection). Filters with other band pass wavelengths may be useddepending upon the type of gas or fluid that is to be detected. The 4.3micron portion of the light enters wafer 69. Virtually all of light 51that impinges pits 72 is reflected as light 80. Light 51 that impingesthe non-etched portions of the second surface of wafer 69 passes throughdetector portions 74 into cavity 78. Pits 72 reflect light 51 so thattemperature sensitive portions 71 are not affected by heat of theincoming light 51. Air and/or gas 67, such as CO₂, flows into andthrough cavity 78 via channels 79. Light 51 is absorbed by CO₂ whichheats up and causes sensors 71 to heat up and result in the detection ofheat and consequently the presence of CO₂, since the wavelength of light51 and the absorption wavelength of CO₂ are the same. As gas 67 passesthrough and is present in cavity 78, light 51 is fluctuating or pulsingin magnitude or intensity and causing the CO₂ of gas 67 to heat andcool. Electrical signals from detector elements 71 go to a processor 81via contacts 75 and leads 60. Processor 81 determines the presence andthe amount of CO₂ and inferentially indicates the presence of toxicgases present in the immediate environment of gas sensor 70. Reflectedlight 80 is kept from sensor elements 71 to minimize fixed signals goingto processor 81. A differential arrangement like that of FIGS. 6a, 6b,6c and 6d may be employed. Alterations of sensor 70 may be made likethose to sensor 10 to directly sense other kinds of gases or liquids.

Gas sensor 70 may be designed to directly detect and indicate thepresence of other gases or liquids besides CO₂. Narrowband pass filter54 would be changed to a filter that would pass a different wavelengthof light 51 which would be equivalent to the absorption wavelength ofthe other kind of gas to be detected and measured. For instance, filterwould be designed to pass 4.6 micron wavelength of light if CO were tobe directly detected by sensor 70 or to a wavelength from 3.2 to 3.4microns if a gas or liquid (VOCs) having hydrocarbon (CH) bonds were tobe directly detected by sensor 70.

FIG. 3 illustrates another opto-thermal gas sensor 82. A siliconsubstrate 83 has etch pits 84. Situated over etch pits 84 arethermoelectric receptors 85. Situated on substrate 83 are spacers 86. Onspacers 86 is a silicon substrate 87. Formed on one surface of substrate87 is a narrow band pass interference filter 88. Formed on the otherplanar surface of substrate 87 is anti-reflective film 89. Formed onfilter 88 are shadow masks 90 which block incoming light coming throughfilm 89, substrate 87 and filter 88 into cavity 91, but only in areasdirectly over thermoelectric sensors 85. The purpose of each shadow mask90 is to largely block light 92 coming into cavity 90 from impinging onsensor elements 85. The source of radiation or light 92 may be from anincandescent light bulb 93 or a microemitter array 94 formed on a sourcesubstrate or wafer 95. Spacers 96 may be formed on substrate 87 or film89 to support substrate or wafer 95 containing light or radiation sourceelements 94. Substrate 95 supported by spacers 96, when formed on wafer87 or film 89, results in a thermal isolation cavity between wafer 95and wafer 87 or film 89.

Light 92 from either microemitters 94 or light bulb 93, is modulatedwith a varying intensity or a pulse waveform. Light 92 goes throughthermal isolation cavity 97 if microemitters 94 are used, or initiallygoes through antireflective film 89 if light bulb 93 is used. Afterlight 92 goes through film 89, substrate 87 and interference filter 88,it enters cavity 86. Light 92 having wavelengths other than theabsorption wavelength of the gas to be detected is blocked by narrowband pass filter 88. Light of all wavelengths is blocked by shadow mask96 to reduce impingement of light 92 on thermal sensors 85. Thermalsensors 85 may be coated with a reflective metal layer to minimizedirect absorption of infrared radiation. Air and/or gas 112 of theambient environment about sensor 82 is free to have a flow 111 in andout of cavity 91. If gas 112 having an absorption wavelength that is thesame as the wavelength of light 92 that passes through filter 88, thenlight 92 is absorbed by that gas 112 and as a result heats up. Theincrease of temperature of gas 112 is detected by thermal sensors 85. Ifthere is no gas having an absorption wavelength which is the same as thewavelength of light 92 passing through filter 88, then there is noabsorption of the light by the gas and no increase or change of thetemperature of the gas and/or air within cavity 91. Therefore, thermalsensors 85 detect no change in temperature. However, if shadow masks 90were not present, then light 92 would impinge thermal sensors 85 whichwould detect increases and/or changes in temperature in cavity 91,thereby providing a large fixed signal in addition to the gas-dependentsignal.

FIGS. 4a, 4b and 4c illustrate the effects of light 92 in cavity 91 withand without shadow mask 90 and metal reflective layers. Waveform of FIG.4a reveals the amplitude of light 92 coming through filter 88 intochamber 91. FIG. 4b shows a signal 99 from thermal sensor 85 when shadowmask 90 is not present. If there is a gas in chamber 91 having anabsorption wavelength, which is the same as that of the light 92 passingthrough interference filter 88, then increased heat in the chamber as aresult of the absorption of light 92 by the gas being detected issuperimposed as curve 100 on curve 99. With the shadow mask 90 in place,and with a reflective layer, signal 99 is largely removed due to theblocking of light 92 from impinging on and being absorbed by thermalsensors 85. The resultant sensor signal with sensor 85 isolation fromlight 92, results in signal 100 shown in FIG. 4c.

Signals from sensors 85 go to amplifier 102 and onto a lock-in amplifier103. Power source 104 outputs an electrical signal 105 which is providedto light bulb 93 or microemitters 94 to result in light 92 of a pulsedor varying intensity. Also, signal 105 is fed to lock-in amplifier 103.A signal output of lock-in amplifier 103 provides an indication of theamount of concentration of the gas detected in cavity 91 and about theambient environment of sensor 82. The signal from amplifier 103 goes toprocessor 106 which inferentially determines from the amount of adirectly detected gas, for example, CO₂, the presence and amounts ofvarious toxic gases that are in the ambient environment immediatelyaround and about the micromachined inferential toxic gas indicator 82.Processor 106 also infers present or past chemical or physical activityaround sensor 82. It also may portend future chemical or physicalactivity. Processor 106 may have a table of information that indicatescertain amounts of concentrations of particular gases or fluids thatinfer the presence of certain amounts of concentrations of other gasesor fluids. The presence of certain amounts of concentrations of othergases or fluids are more accurately inferred by the presence of certainamounts of concentrations of the particular gases or fluids in cavity 91because the amounts of the detected concentrations, such as CO₂, are upto several magnitudes larger than the certain amounts of concentrationsof the other inferred gases or fluids. Sensor 82 has processor 106connected to it via amplifiers 102 and 103, such that a signal fromthermal sensors 85, indicates an amount of concentration of gas 112 orfluid having an absorption wavelength at a first wavelength, and goes toprocessor 106. Processor 106 processes the signal from sensors 85 andprovides inferred information indicative of a presence of other gases orfluids and/or future or present or past chemical or physical activity.This information is achieved by a table of information in processor 106,which indicates that certain amounts of concentrations of particulargases 112 or fluids infer the presence of certain amounts ofconcentrations of other gases or fluids. The presence of certain amountsof concentrations of other gases or fluids is more accurately inferredby the presence of certain amounts of concentrations of the particulargases 112 or fluids, since the latter amounts of concentrations are upto several magnitudes larger than the certain amounts of concentrationsof the other gases or fluids. For example, the first wavelength may beat an absorption wavelength of CO₂, and thus the presence of CO₂ wouldindicate the presence of much smaller quantities of certain combustionproducts.

FIG. 5 shows the fabrication of thermo-electric sensor 85. Siliconsubstrate 83 has an edge pit 84 for purposes of thermal isolation ofdetector 85. A micromachined array of thermal electric sensors 85 areformed from overlapping thin film metals 107 and 108. They are formedbetween layers of silicon nitride 109 which is formed on siliconsubstrate 83. The sensor portion of metal layers 107 and 108 areisolated to the areas of overlap and contact between metals 107 and 108,which are situated over etch pits 84, which cuts 110 define. A metalreflective layer (gold) may be applied (109a) to reduce directabsorption of radiation by the thermo-electric sensor 85.

I claim:
 1. A gas/fluid sensor comprising:a substrate having a pluralityof pits; an array of thermoelectric sensors formed on said substrateover the plurality of pits; a narrow band pass filter proximate to andat a first distance from said array of thermoelectric sensors resultingin a space between said array of thermoelectric sensors and said narrowband pass filter; a shadow mask formed on said narrow band pass filter;and an external radiation source situated proximate to said narrow bandpass filter.
 2. The gas/fluid sensor of claim 1 wherein:said radiationsource can emit radiation to go through said narrow band pass filter; afirst portion of the radiation is blocked by said shadow mask to preventthe radiation from impinging on said array of thermoelectric sensors;the space between said array of thermoelectric sensors and said narrowband pass filter is filled with a gas or fluid from an ambientenvironment around the gas/fluid sensor; and a second portion of theradiation, not blocked by said shadow mask, will go into the spacebetween said array of thermoelectric sensors and said narrow band passfilter, and if not absorbed by a gas or fluid, will impinge areas ofsaid array of thermoelectric sensors that have no thermoelectricsensors.
 3. The gas/fluid sensor of claim 2 wherein:said radiationsource, during emission, emits radiation having an intensity thatvaries; said narrow band pass filter passes radiation at a firstwavelength; if the gas or fluid in the space between said array ofthermoelectric sensors and said narrow band pass filter has anabsorption wavelength at the first wavelength, then the gas or fluidabsorbs the radiation and changes in temperature according to avariation of the intensity of the radiation; if the gas or fluid in thespace between said array of thermoelectric sensors and said narrow bandpass filter has an absorption wavelength not at the first wavelength,then the gas or fluid does not absorb the radiation and changes intemperature by a substantially fixed amount according to a variation ofthe intensity of the radiation, and will vary in temperature by anincreased amount if the gas or fluid does have an absorption wavelengthat the first wavelength; and said array of thermoelectric sensors detectthe temperature of the gas or fluid in the space between said array ofthermoelectric sensors and said narrow band pass filter, and output asignal indicative of the temperature.
 4. The gas/fluid sensor of claim 3further comprising:a modulated electrical power signal source connectedto said radiation source; a lock-in amplifier having a first inputconnected to said array of thermoelectric sensors, a second inputconnected to said modulated electrical power signal source, and havingan output that can provide a signal indicating an amount ofconcentration of a gas or fluid having an absorption wavelength at thefirst wavelength, in the space between said array of thermoelectricsensors and said narrow band pass filter, and in turn in the ambientenvironment around the gas/fluid sensor.
 5. The gas/fluid sensor ofclaim 4 further comprising a processor connected to the output of saidlock-in amplifier wherein said processor can infer the presence ofamounts of concentrations of other gases and/or fluids based on thesignal at the output of said lock-in amplifier, for a given firstwavelength.
 6. A gas/fluid sensor comprising:a substrate having aplurality of pits; an array of thermal sensors formed on said substrateover the plurality of pits; a narrow band pass filter proximate to andat a first distance from said array of thermal sensors resulting in aspace between said array of thermal sensors and said narrow band passfilter; radiation blocking areas formed on said narrow band pass filter;and an external radiation source situated proximate to said narrow bandpass filter.
 7. The gas/fluid sensor of claim 6 wherein:said radiationsource can emit radiation to go through said narrow band pass filter; afirst portion of the radiation is blocked by said radiation blockingareas to prevent the radiation from impinging on said array of thermalsensors; the space between said array of thermal sensors and said narrowband pass filter is filled with a gas or fluid from an ambientenvironment around the gas/fluid sensor; and a second portion of theradiation, not blocked by said radiation blocking areas, will go intothe space between said array of thermal sensors and said narrow bandpass filter, and if not absorbed by a gas or fluid, will impinge areasof said array of thermal sensors that have no thermal sensors.
 8. Thegas/fluid sensor of claim 7 wherein:said radiation source, duringemission, emits radiation having an intensity that varies; said narrowband pass filter passes radiation at a first wavelength; if the gas orfluid in the space between said array of thermal sensors and said narrowband pass filter has an absorption wavelength at the first wavelength,then the gas or fluid absorbs the radiation and changes in temperatureaccording to a variation of the intensity of the radiation; if the gasor fluid in the space between said array of thermal sensors and saidnarrow band pass filter has an absorption wavelength not at the firstwavelength, then the gas or fluid does not absorb the radiation andchanges in temperature by a substantially fixed amount according to avariation of the intensity of the radiation, and will vary intemperature by an increased amount if the gas or fluid does have anabsorption wavelength at the first wavelength; and said array of thermalsensors detect the temperature of the gas or fluid in the space betweensaid array of thermal sensors and said narrow band pass filter, andoutput a signal indicative of the temperature.
 9. The gas/fluid sensorof claim 8 further comprising:a modulated electrical power signal sourceconnected to said radiation source; a lock-in amplifier having a firstinput connected to said array of thermal sensors, a second inputconnected to said modulated electrical power signal source, and havingan output that can provide a signal indicating an amount ofconcentration of a gas or fluid having an absorption wavelength at thefirst wavelength, in the space between said array of thermal sensors andsaid narrow band pass filter, and in turn in the ambient environmentaround the gas/fluid sensor.
 10. The gas/fluid sensor of claim 9 furthercomprising a processor connected to the output of said lock-in amplifierwherein said processor can infer the presence of amounts ofconcentrations of other gases and/or fluids based on the signal at theoutput of said lock-in amplifier, for a given first wavelength.
 11. Amicromachined integrated circuit gas/fluid sensor comprising:a firstwafer having a first surface; a narrow band pass filter for a firstwavelength formed on a second surface of said first wafer; a secondwafer having a first surface proximate to said narrow band pass filter;a plurality of pits formed on a second surface of said second wafer,wherein each pit of said plurality of pits substantially reflectsradiation impinging the pit; a plurality of areas formed in the secondsurface of said second wafer, wherein each area substantially transmitsradiation of the first wavelength; a plurality of thermal sensorsproximate to the second surface of said second wafer wherein eachthermal sensor of said plurality of thermal sensors is situated over apit of said plurality of thermal sensors; a third wafer having a firstsurface at a distance from but adjacent to said plurality of thermalsensors and the second surface of said second wafer to form a cavityhaving an opening so that a gas or fluid from the ambient environmentabout the sensor may enter and/or exit the cavity; and an externalradiation source situated proximate to said first wafer.
 12. The sensorof claim 11 further comprising a through-the-wafer contact formed on thefirst surface of said first wafer and through said first and secondwafers to said plurality of thermal sensors.
 13. The sensor of claim 12further comprising an antireflective coating formed on the first surfaceof said first wafer.
 14. The sensor of claim 13 wherein:radiation thatmay come from said radiation source and pass through the antireflectivecoating, said first wafer, said narrow band pass filter, a first portionof the radiation from said narrow band pass filter will be reflected bysaid plurality of pits formed on the second surface of said secondwafer, and a second portion of the radiation from said narrow band passfilter will pass through said second wafer where none of said pluralityof pits is formed; gas or fluid in the cavity impinged by radiationpassing through said second wafer will change in temperature if the gasor fluid has an absorption wavelength at the first wavelength; gas orfluid in the cavity impinged by radiation passing through said thirdwafer will change in temperature by a substantially fixed amount if thegas or fluid has an absorption wavelength not at the first wavelength,and will vary in temperature by an increased amount if the gas or fluiddoes have an absorption wavelength at the first wavelength, and willvary in temperature by an increased amount if the gas or fluid does havean absorption wavelength at the first wavelength; and said plurality ofthermal sensors can sense a change of temperature of the gas or fluid.15. The sensor of claim 14 wherein:said plurality of thermal sensorsoutputs a signal that indicates a magnitude of the change of temperatureof the gas or fluid; and the magnitude of the change of temperatureindicates an amount of concentration of the gas or fluid in the cavityand in turn in the ambient environment of the gas/fluid sensor.
 16. Thesensor of claim 15 further comprising a processor wherein:the signalfrom said plurality of thermal sensors, indicating an amount ofconcentration of the gas or fluid having an absorption wavelength at thefirst wavelength, goes to said processor; and said processor processesthe signal from said plurality of thermal sensors and provides inferredinformation indicative of a presence of other gases or fluids and/orfuture or present or past chemical or physical activity.
 17. The sensorof claim 16 wherein:said processor comprises a table of information thatindicates certain amounts of concentrations of particular gases orfluids which infer the presence of certain amounts of concentrations ofother gases or fluids; and the presence of certain amounts ofconcentrations of other gases or fluids is more accurately inferred bythe presence of certain amounts of concentrations of the particulargases or fluids because the latter amounts of concentrations are up toseveral magnitudes larger than the certain amounts of concentrations ofother gases or fluids.
 18. The sensor of claim 17 wherein:the firstwavelength is at an absorption wavelength of CO₂ ; and the presence ofCO₂ indicates the presence of certain combustion products.